High voltage transmission lines require permanent current or voltage monitoring in order to react to faults in a proper manner. Specifically, the ability to quickly and reliably detect current transients is paramount because protection circuits must react in adequate time to mitigate short circuits that can damage equipment located at either end of a transmission line.
In some applications, typically requiring high voltages (>69 kV), current flow (steady state or transient) can be measured using traditional iron core current transformers or using optical current transducers. Iron core transformers include costly ceramic insulators that are used to provide adequate isolation between the transformers' windings. As such, they may not be an economical solution when considering deployment in large electricity distribution networks. Most importantly, however, these transformers have undesired performance characteristics. Namely, they can introduce significant distortions in the measured current signal due to the hysteresis of their iron cores.
Fiber-optic current transducers (FOCTs) have been used as an alternative technology to circumvent the aforementioned issues; FOCTs can be less costly, although for applications with voltages lower than 15 kV, this may not be the case. Nevertheless, FOCTs have superior performance. They operate on the principle of Faraday rotation, which is a magneto-optical effect whereby a rotation of the plane of polarization of a light beam confined in a fiber-optic waveguide placed near the transmission line is observed in response to a magnetic field induced by the flow of current. The rotation angle is linearly proportional to the component of the magnetic field in the direction of propagation of the light, and as such, the change in angle can be correlated with the strength of the magnetic field, which can in turn be used to calculate the current.
A typical reflection-based FOCT measurement sensor includes a module of optical components with three fibers attached thereto. Two of the fibers (data fibers) are connected to a receiver, and the third fiber is a low birefringence fiber (LBF) that is wrapped around the transmission line.
Inside the module, there is a polarizer that polarizes the light at zero degrees. After the light is polarized, it is rotated by 22.5 degrees using a Faraday rotator. From the rotator, the light enters the LBF wherein it is further rotated by the magnetic field induced by the current in the transmission line. At the end of the LBF, there is a mirror that reflects the light back to the rotator, which rotates the light another 22.5 degrees. After the second rotation, the light is broken into two components that are transduced by the receiver into two electrical signals denoted “X” and “Y.”
The Faraday rotator may cause significant errors in the X and Y components. These errors are manifested as DC offsets in each of the signals, and they are due to variations in temperature at the crystal that make up the Faraday rotator. As such, these offsets are termed “crystal offsets.” Offsets may also arise from losses that occur when light travels through the data fibers. All of these offsets can introduce significant errors in estimating the current in the transmission line.